People who lose a leg today may be in a bad situation. Some days, a simple staircase may seem like an insurmountable challenge. Walking up a grassy slope is too difficult to attempt, because multiple falls may be inevitable. War, accidents and disease keep this disadvantaged population growing. Prosthetics, or synthetic replacements for missing anatomical structures, hold the promise of restoring some of this lost function and improving quality of life.
Just trying to regain functional mobility, amputees spend an average of $8,000 on below-knee (BK) prosthetic legs that last three to five years. Rather than spend this money on costly, non-repairable devices, one hundred and twenty thousand American amputees have chosen crutches or wheelchairs, and they won't walk again.
Just as the speed of a vehicle is maintained through regular energetic pushes received from pistons firing in the engine, normal human gait relies on well-timed pushes from the anatomy of the foot, during the toe-off portion of the walking cycle. Providing a suitable timing of toe-off, while providing a stable, level base—a preferable innovation addressed in this application—is lacking from existing feet prostheses and may be relevant for natural and comfortable ambulation.
The human gait is in reality a very complex process that at a basic level may be described as a series of repeating operations carried out by a single leg: 1) initial heel strike, 2) double support as both feet contact the ground, 3) stance phase as one leg supports the entire body weight, 4) pre-swing or heel-rise as the heel rises from the ground, 5) toe-off as the moment that the toes lose contact with the ground, and finally 6) swing phase, where the leg, acting as a pendulum, comes forward in preparation to repeat the process. In a two legged description of pre-swing, the heel of the contralateral leg strikes the ground at the exact moment that the ipsilateral heel rises. This is called double stance phase, and may be relevant to understanding the innovations presented in this application. Coordinated movement between the legs and the overall balance and trajectory of the body dynamic may be also relevant to successful ambulation.
Currently, there are two dominant paradigms of prosthetic foot design: post-like, conventional feet (CF) and leaf-spring-like, energy storing feet (ESF). Both of these designs change shape under loading, in an attempt to mimic the human foot. The classic CF foot, also known as the Solid Ankle Cushioned Heel (SACH), foot may provide a stable base for support, and is functionally unchanged since its conception in the 1960's. Introduced in the 1980's, carbon-fiber, leaf-spring ESF designs allow amputees to run by mimicking the ankle plantar flexors, returning energy to their stride. Para-lympic records rivaling their Olympic counterparts show that the ESF paradigm works very well for running, but studies have failed to show that these benefits extend to walking. 40% of transtibial amputees do not use prostheses and 78% of transfemoral amputees forego this intervention. Thus, over 120,000 amputees do not use prosthetic legs, preferring wheelchairs or crutches, never walking again. Studies of amputee psychosocial adjustment have linked positive emotional coping and higher levels of physical independence.
Depending on the type of foot used, CF or ESF, and the specific manufacturer, there have been subtle but significant differences in parameters such as stride length, symmetry of stride, and timing of the various phases of gait. For either foot type, stride length is shorter for strides where the prosthesis is the supporting limb, gait symmetry is markedly decreased, and the timing of the phases of gait may be disrupted. Most notably, there is a shortened stance phase on the prosthesis, a late toe off, and a longer swing phase on the affected side as well. Studies also describe an early incidence of low back and patellar-femoral osteoarthritis in unilateral amputees. The literature clearly shows that current prostheses fail to walk like an intact limb. In fact, clinical prostheticists have expressed the opinion that some “middle ground” between the unsophisticated CF feet and the highly athletic ESF feet is needed. Embodiments of the invention outlined here may be just that middle ground.
To lay the foundation for the rest of this submission, a few questions may be asked. Precisely how may an intact limb walk? And what is the role of the foot in this process? To address the first question, this application may present two different types of engineering control systems, and may provide illustrative examples. To address the second question, more studies may be presented, furthering the discussion, showing results of highly detailed, instrumented gait studies of the foot. Comparisons between the functional movements of the human foot, and the functional movements of current prostheses may follow. The improvements embodied in embodiments of the proposed device may address many of the shortcomings seen in the current technology.
With all of the myriad muscles and bones in human hips, legs, and feet, there is no “right” answer for how to propel one's self across a room or up a slope; however, there may be more optimal solutions, for example, ones that may be less abusive to the anatomy and/or ones with more optimal energetic efficiency. Early incidence of osteoarthritis, a degenerative joint change, is one indicator of a suboptimal movement strategy.
There may be many ways to walk, and data shows that people don't walk in exactly the same way with each stride. The hips may work harder on some strides than others; sometimes the lower leg may contribute varying amounts torque to the stride. Walking from one's hips may be described as a “top-down” control mechanism, where forces from the proximal leg may dictate the position and accelerations of the distal structures. This mechanism is very clearly illustrated in above knee (AK) amputees. Until recent, expensive innovation of computer controlled knees, AK amputees who wanted to walk faster than the return rate of their knee spring had to use a “hip snap,” flinging their prosthesis out quickly with their hip flexors, and then quickly contracting their hip extensors to snap the prosthetic knee straight in time for heel-strike. Thus, the anatomic ranges of motion guided the position of the prosthetic anatomy, but the timing the movement was controlled by the hip, in a “top-down” fashion.
A “top-down” control mechanism may also be seen in studies of trans-tibial amputees. The iEMG data of one study showed a greater use of the biceps femoris (BF) as compared to the antagonistic vastus medialus (VM) in the amputated limb, as opposed to the normal limb. The mean ratios of BFNM activity during the first half of stance phase was 3.8 in the amputated limb and 2.0 in the sound leg, with a P value of less than 0.042. Furthering elaboration on the “top-down” nature of this control system, an exceptionally statistically rigorous study from 2002 revealed some interesting trends in the flexor/extensor ratios for the knees of unilateral, trans-tibial amputees, as compared to normal volunteers. Though the amputees were much weaker than the normal control group, this study showed that there was no significant difference between the knee flexor/extensor ratios for peak bending moment, total work, or maximum power comparing either leg of the amputees and either leg of the non-amputees. Of course, the BF and VM may be also knee flexors and extensors, but not during the relevant time-span cited by the first study, early stance phase. Considering these studies together, one may conclude that trans-tibial amputees use the hip of the amputated leg more than the hip of their sound leg, and that they use their knee flexors and extensors normally. Clearly, the control mechanism being employed in a trans-tibially amputated limb is “top-down.”
The overuse of a particular muscle must result in overuse of the surrounding and supporting muscles. For example over loading a hip muscle causes the hip stabilizers to be over-recruited. If multifidus and transversus abdominus, the deepest pelvic stabilizers, may be overwhelmed, the larger quadratus lomborum (OL) and erector spinae (ES) muscles that may be normally used for motion may be recruited to help it. When the QL and ES are used as stabilizers, the agonists may also be recruited as stabilizers, just as transverses abdominus is recruited along with multifidus. When the QL and ES become a routine part of the stabilizing muscle pattern, they become tonic and rigid. Thus, putting a great deal of compression on the spine. This is a well-known pattern of muscle use and, if allowed to progress unchecked, may eventually result in degenerative joint changes in the lower spine.
Walking from the foot, as opposed to the hip, may be modeled as a “bottom-up” control scheme, where the distal anatomy directs the position of the proximal anatomy. The coordination of the metatarsophalangeal joint (MTP) of the great toe and the subtalar joint may create a dynamic in gait where the proximal foot and tibia subtly change angular position. This angular change may be the start of building momentum for toe off. In context of the gait cycle, starting from single stance phase, as the tibial shaft moves past perpendicular and over the foot, the subtalar joint may be eccentrically loaded. This may be seen as a “flatter” transverse arch. This subtle motion may progress with the tibial shaft advancement, with a maximum angular change of 10 degrees. In double stance phase, much of this weight may be off-loaded to the other leg, but the transverse arch may not yet spring back into shape. In fact, this new conformation may be maintained until just after heel rise. When the heel leaves the ground, passing the remaining force loading to the ball of the great toe, the MTP of the great toe may be forced into extension. This motion may pull on the plantar aponeurosis, which in turn may pull on the calmayeus and the Achilles tendon. This action may loft the transverse arch back to its stance phase conformation, subtly altering the position of the ankle and the tibia, and thus may change position of the knee and hip.
The relevant anatomy for this coordination of the first MTP and subtalar joints is well documented. The plantar aponeurosis spans both joints, as may the tendon of the flexor hallucis longus. Different research references attribute this coordination to each of these sources. The action of arching the subtalar joint by forcibly extending the first MTP has been described as the Windlass mechanism, and this passive, non-muscular change may be a function of timing and anatomic length. This timing may be influenced by the peronii, the tibialis anterior, and the intrinsic foot muscles. Of course, a passive prosthesis may not duplicate the action of these muscles, but it may mimic the action of the plantar aponeurosis. Due to the quasi-psuedoviscoelastic nature of the plantar aponeurosis and the surrounding musculature, this quick lofting of the plantar arch may be an energy storage mechanism. The energy may then be released, a moment later, on toe off. As seen in the temporal gait asymmetry of amputees, most notably in late stance and swing phases, studies have shown conclusively that this action is not accomplished in either CF or ESF designs.
These two distinct “ways of walking” represent extremes, and, as human nature dictates, we all walk with a varying degree of each mechanism. Amputees must rely exclusively on the strategy of top-down control, resulting in an overcompensation of the remaining anatomy which in turn may cause early degenerative changes. What is needed is a prosthesis that accurately imitates the relevant biomechanics of the natural foot, allowing for the contributions of the more efficient “bottom-up” gait style.
There is a definite coordination between the joints of the foot. The angular relationship shown between the forefoot and hallux may be the angular position of the first MTP. The angular motion between the forefoot and hindfoot may reflect the motion of the subtalar joint. A few studies have explored the detailed biomechanics of the foot using this powerful analytical technique, but they did not combine the detailed foot analysis with the protocol for the rest of the body. Thus, no quantified joint powers were generated. Experts may also be aware of the subtle, but highly significant errors in instrumented gait analysis of ESF prosthesis gait. Failure to accurately model the center of curvature of the leaf spring foot, for the purpose of reverse engineering the joint torques, may be the documented source of this error. The standard seven segment lower body model, used to reverse engineer joint torques, may use a rigid single segment foot. This simplified model may leave out both the first MTP and the subtalar joints, masking the relevant contribution of the Windlass mechanism, a subtle “bottom-up” contributor of gait mechanics. Theoretically, a nine segment lower body model, as seen in computer simulations, may show sensitivity to changes in spring stiffness of the MTP joint at push off, but still may exclude the subtalar joint or any coordination of the two.
The movement of the subtalar joint and first MTP during stance phase and toe-off, as described above, may correlate to a relatively new area of prosthetics research. Roll-over shape may be defined as the geometry a foot/ankle complex takes during the single limb stance phase of walking. As the center of weight may pass over the long axis of the prosthetic foot, it may bend according to its stiffness. The shape described by this bending may be the rollover-shape, and it may be defined in general terms as a rigid rocker model of the foot/ankle complex. A three dimensional rollover shape may be called a rollover surface, and a two dimensional shape may be called a rollover profile.
Studies of various prosthetic feet with the rollover profile methodology have shown that the “effective foot length” during walking is surprisingly short in many cases. For example, a size 28 cm SACH foot may display a functional length of less than 20 cm. The length of the rollover profile is significant for many gait parameters, and recent studies show that it may be relevant to how much oxygen is consumed during gait.
Considering the rollover profile length, along with the recent research into oxygen consumption dynamics, points toward a discrepancy that may be more significant than previously thought. In fact, the energy used in walking may be proportional to the fourth power of the step length. Since the stride length may be equal to the functional foot length plus the distance covered by swing phase, feet with shorter rollover profiles may deliver shorter stride lengths. The average step length is about 0.75 meters, and the difference in rollover profile between a SACH foot and a flex-foot is about 6 centimeters. Considering the relationship described above, one would anticipate a large energy savings by using the longer flex-foot, because the step length is almost 10% greater for the ESF versus over the CF. Surprisingly, this energy savings is not seen in any ESF models with longer rollover profiles. In fact, research shows a small energy savings, on the order of 3%, and some of the research subjects in that study found that some ESF feet were more tiring to use than some CF feet. This correlates well with the experience of clinical prosthetists, who describe that their patients often work against their ESF feet, because their return of power is not biomechanically accurate. Indeed, studies of prostheses show that a very small component of this energy return is in the antero-posterior direction, unlike the natural human limb.